1. Field of the Invention
This invention relates to a chirp spread spectrum signal multiple access apparatus in which a signal multiple access is carried out by offsetting a plurality of signals in view of time under application a chirp signal as a spread spectrum code.
2. Brief Description of the Related Art
As the prior art signal multiple access system using spread spectrum, it is possible to provide a code division multiple access communication apparatus disclosed in a gazette of Japanese Patent Laid-Open No. Hei 7-50649, for example. In this code division multiple access communication apparatus, an information code is processed in direct-spread with a spread code and its major constitution is as indicated in FIG. 8.
In the code division multiple access communication apparatus shown in FIG. 8, the multiple access number (n) is defined as 2 and a symbol interval of the information code is defined as Ts. A transmission side of the code division multiple access communication apparatus is comprised of information code generators 201, 202, spreading circuits 203, 204, spread code generator 205, delay circuit 206, adder 207 and modulator 208. At the receiving side, it is comprised of a de-spreading filter 210, delay circuit 211 and samplers 212, 213.
At the receiving side of the code division multiple access communication apparatus, transmission code generated at the information code generator 202 is convolved by the spreading circuit 204 with a spreading code outputted from the spread code generator 205 so as to spread it. In addition, at the transmission side of it, the transmission code generated from the information code generator 201 is also convolved with the spread code outputted from the spread code generator 205 so as to spread it.
If the outputs from the spreading circuits 203, 204 are added and made composite by the adder 207, an output from the spreading circuit 203 and an output from the spreading circuit 204 can not be discriminated at the receiving side and further an output from the information code generator 201 and an output from an information code generator can not be separated at the receiving side. In view of this fact, at the transmission side, a delay spreading code in which a spreading code outputted from the spreading code generator 205 is delayed only by Ts/2 with the delay circuit 206 is supplied to the spreading circuit 203.
Output signals from the spreading circuit 203 and the spreading circuit 204 are added by the adder 207 and outputted as one adding signal. At the modulator 208, modulation for converting a signal into a signal format which can easily be transmitted in respect to the adding signal is carried out and then a modulation signal is transmitted and outputted from the modulator 208.
In turn, at the receiving side, a correlative calculation is carried out with a demodulation signal and the same spreading code as the spreading code generated by the spreading code generator 205 by the inverse spreading filter 210 having a modulation signal received and inputted thereto. Filtering signals outputted from the inverse spreading filter 210 are displaced by Ts/2 and divided into two segments, respectively, one of the segments is extracted by a sampler 212 and the other segment is delayed by the delay circuit 211 only by Ts/2, thereafter it is extracted with the sampler 213 at the same timing as that for the sampler 212 so as to attain a sample signal supplied to the demodulation receiving processing.
In this way, if the timings for the spreading codes are displaced to perform a signal multiple access processing, it becomes possible to perform a faster signal transmission under the same frequency range width.
A case in which a signal multiple access processing with a direct spreading using the aforesaid spreading code will be studied in reference to a case in which it is applied to a portable terminal and the like, for example. In general, a chip rate of the spreading code in the direct spreading system is set to be more than ten times of a symbol rate (1/Ts) of the information code, although due to this fact, there may occur a problem that a calculation speed of the spreading circuit may become remarkably fast and a power consumption of element is increased. In particular, since the portable terminal uses battery cells as its power supply, it is difficult to employ a communication system showing a high power consumption.